He was recommended by d’Alembert for his post at the École Militaire, where Laplace was a colleague. Condorcet encouraged him to write what became the standard textbook on geometry for the next century, succeeding Euclid’s ‘Elements’. Germain wrote to him about number theory, their ensuing correspondence amounting to a collaboration. He encouraged Cauchy’s geometrical work and praised Jacobi’s advances with elliptical functions. Abel described him as “extremely amiable, but unfortunately as old as the stones”, while Poisson said that his very modest colleague only wanted to be spoken about in respect of his work.